When turbomachines such as, for example, gas turbine plants are in operation, combustion-driven thermoacoustic oscillations often occur in the combustion chambers, these taking the form of fluidic instability waves at the burner and lead to flow vortices which greatly influence the entire combustion operation and lead to undesirable periodic heat releases within the combustion chamber. This results in pressure fluctuations of high amplitude which may lead to undesirable effects, such as to a high mechanical load on the combustion chamber housing, to increased NOx emission as a result of inhomogeneous combustion or even to an extinguishing of the flame within the combustion chamber.
Thermoacoustic oscillations are based at least partially on flow instabilities in the burner flow which are manifested in coherent flow structures and which influence the mixing operations between air and fuel.
A series of techniques have become known in the meantime for counteracting thermoacoustic oscillations, for example with the aid of a cooling-air film which is conducted over the combustion chamber walls or by means of an acoustic coupling of what are known as Helmholtz dampers in the region of the combustion chamber or in the region of the cooling-air supply.
It is known, furthermore, that the combustion instabilities occurring in the burner can be counteracted by the fuel flame being stabilized by the additional injection of fuel. Such an injection of additional fuel takes place via the head stage of the burner, in which a nozzle lying on the burner axis is provided for the pilot fuel gas supply, although this leads to an enrichment of the central flame stabilization zone. However, this method of reducing thermoacoustic oscillation amplitudes entails the disadvantage that the injection of fuel at the head stage is accompanied by an increase in the emission of NOx.
Investigations of the formation of thermoacoustic oscillations have shown that flow instabilities often lead to these instabilities. Particular importance is attributed, in this case, to the shear layers which form between two mixing flows and which initiate waves running perpendicularly to the flow direction (Kevin-Helmholtz waves). These instabilities on shear layers, in combination with the combustion process which is taking place, are mainly responsible for the thermoacoustic oscillations triggered by reaction rate fluctuations. Where a burner of the abovementioned type is concerned, these largely coherent waves lead, under typical operating conditions, to oscillations with frequencies in the range around 100 Hz. Since this frequency coincides with typical fundamental characteristic modes of many annular burners in gas turbine plants, the thermoacoustic oscillations present a problem. More detailed statements in this respect may be gathered from the following publications: Oster & Wygnanski 1982, “The forced mixing layer between parallel streams”, Journal of Fluid mechanics, Vol. 123, 91-130; Paschereit et al. 1995, “Experimental investigation of subharmonic resonance in an axisymmetric jet”, Journal of Fluid Mechanics, Vol. 283, 365-407; Paschereit et al., 1998, “Structure and Control of Thermoacoustic Instabilities in a Gas-turbine Burner”, Combustion, Science & Technology, Vol. 138, 213-232).
As may be gathered from the foregoing publications, it is possible to influence the coherent structures forming within the shear layers by the specific introduction of acoustic excitation in such a way that the formation of such vortices is largely prevented. Fluctuations in the heat release are consequently forestalled and the pressure fluctuations reduced.
Premixed flames require zones of low velocity, in order to become stabilized. For stabilizing the flame, there are backflow zones which are generated either by the wake downstream of disturbance bodies or by aerodynamic methods (vortex breakdown). The stability of the backflow zone is a further criterion for the stability of combustion and for the avoidance of thermoacoustic instabilities.